If only there were a snappy term for the impact a sudden summer scorcher has on the human body.

Such as heat lag.

Just as flying across half a dozen time zones leaves most people feeling groggy and sluggish, abrupt immersion in 90-degree weather and matching humidity takes a toll. The body’s not ready. It needs time to adjust.

“Even under the best circumstances, it takes two to three weeks to get used to heat,” said Tim Church, medical director of the Cooper Institute in Dallas.

Changes happen on multiple physiological levels, so that the body is able to sweat more and with increased effectiveness – excreting less salt – and cells get used to working at higher temperatures.The body’s adjustment to extended heat exposure results in a lower heart rate even during minor exertion, though those gains are lost once temperatures cool.

It’s as if the body is in training. The experts call it acclimatizing.

With soldiers serving in harsh desert conditions, adaptation is a significant concern for the military. “Heat strain is probably the most pervasive strain you can encounter,” Sawka said.

During extreme heat, the body is greatly challenged to maintain a constant internal temperature. If the air is saturated with moisture, the ability to dissipate heat through sweating is compromised. The sweat simply does not evaporate.

The result is physical stress. The pulse rises. Salt levels plunge. Blood may circulate more slowly to vital organs. When ozone levels rise and the air is bad, the lungs are stressed, too.

A lack of acclimatization explains why heat waves early in the summer are deadlier than later ones. Deaths tend not to occur on the first hot day but on subsequent ones.

“More people die of heat in May and June than in August, even if the weather is exactly the same,” said Laurence Kalkstein, who as a University of Delaware bioclimatologist researches the effect of heat on human health.

In cities with relatively variable climates, heat waves are more dangerous than they are in those with consistent heat to which people become accustomed.

The National Weather Service recently relied on Kalkstein’s research in establishing a system to issue heat-wave warnings in 20 cities. Forecasters do not rely only on predicted heat and humidity to decide whether to tell people and governments to take precautions. They also plug in wind speed, cloud cover, successive days of heat, deaths during past heat waves and availability of air conditioning to obtain a computer-guided risk assessment.

“Heat is kind of an underrated killer,” Weather Service meteorologist Mark Tew said. “More people die of heat-related mortality than any other (environmental) factor. It’s not as sexy as tornadoes or other things.”